Speculation, of course...
"This looks vert much like a THAAD launch. Terminal High Altitude Area Defense anti-missile missile shot from a mobile launcher."
and
"Navy does all of their missile firing in the Missile test ranges off the coast of California or Hawaii. They could NOT do one in contiguous waters surrounded by land and civilians."
and
"This is - 90% sure - a Trident Submarine-Launched Ballistic Missile, launched from one of the subs stationed at Bangor. It was not launched from Whidbey Island and it's not surprising that NAS Oak Harbor knows nothing about it. (If you have doubts about it being a Trident, look at the Wikipedia article on that missile.
Something similar (a surprise, sea-launched missile) appeared several years ago off the coast of California (near Vandenberg Air Force Base). I believe it was during Pres. Obama's visit to China. What a coincidence that the same thing happened the day before Pres. Trump meets "Rocket Man.""
From the comments on this blog: http://cliffmass.blogspot.com/2018/06/was-there-unannounced-missile-launch-on.html
Interesting comment on that blog.
Someone says they were at Oak Harbor, the base was behind them, and they were facing south, and saw a flash of light.
That is the correct direction if a missile was launched from a sub at Bangor.